1. Field of the Invention
The present invention relates to a hydraulic tensioner used for applying an appropriate tension to a timing belt or a timing chain of a vehicle engine.
2. Description of the Related Art
Conventionally, a timing belt or a timing chain is used in a drive system for transmitting rotational motion from a crankshaft to a camshaft of an automobile engine, and a hydraulic tensioner is widely used to suppress vibration, which would otherwise occur in the timing belt or chain when the timing belt or chain travels, and to maintain a proper tension in the timing belt or chain.
FIG. 4 schematically illustrates a chain transmitting device for an engine in which a hydraulic tensioner is used. The hydraulic tensioner 41 is mounted on an engine body on the slack side of a chain (timing chain) 62. The chain 62 is fitted around and extends between a driving sprocket 59 rotated by a crankshaft 58 of the engine and a driven sprocket 61 fixed on a camshaft 60.
The hydraulic tensioner 41 includes a plunger 43 projecting from the front face of a tensioner housing 42 such that the plunger 43 can undergo reciprocating motion. A tip portion of the plunger 43 presses the rear side of a tension lever 64 at a position near the free end thereof. The tension lever 64 is pivotally supported on the engine body by means of a support shaft 63. Through such an arrangement, a shoe surface 65 of the tension lever 64 is brought into slidable contact with the slack side of the chain 62 to thereby apply tension to the chain 62.
Most conventional hydraulic tensioners have a relief valve or an orifice provided alone. The hydraulic tensioner equipped with only the relief valve has a problem that air staying in the plunger cannot escape until the air pressure goes up to a predetermined pressure. On the other hand, the hydraulic tensioner equipped with the orifice alone would encounter a problem that the hydraulic pressure is likely to becomes extremely high or extremely low, making it difficult to achieve an accurate pressure setting. To deal with these problems, attempts have been made to provide hydraulic tensioners equipped with both a relief valve and an orifice.
One example of the proposed hydraulic tensioners is shown in FIG. 5, which includes a relief valve and an orifice. provided seperately, as discussed below.
The hydraulic tensioner 41 includes a plunger 43 slidably mounted in a housing 42 so that a high-pressure oil chamber 44 is defined between the housing 42 and an inner part of the plunger 43. The plunger 43 is urged in a direction to project from the housing 42 by means of a spring 45. A tip end of the plunger 43 is provided with an end plug 55.
Between the housing 42 and the high-pressure oil chamber 44, there is provided a check valve mechanism 47 including a check ball 46. The check valve mechanism 47 permits oil to flow into the high-pressure chamber 44 while blocking the oil from flowing out from the high-pressure oil chamber 44. The high-pressure oil chamber 44 is always filled with the oil which is supplied to an oil supply passage 48 from an oil supply source (not shown) via the check valve mechanism 47.
The plunger 43 is also formed with a secondary oil chamber 50 communicating with the high-pressure oil chamber 44 through an oil passage 49. A sleeve 51 is slidably received in the secondary oil chamber 50 and urged in an axial inward direction by a spring 52. The plunger 43 further has a discharge port 53 normally closed by the sleeve 51. The discharge port 53 is opened when the sleeve 51 is displaced in an axial outward direction past the discharge port 53 against the force of the spring 52. The spring-loaded sleeve 51 assembled in the plunger 43 forms a relief valve 54.
The housing 42 of the tensioner 41 further includes an orifice member 56 having a peripheral helical orifice 56A designed to communicate with an upper part of the high-pressure oil chamber 44 for allowing the leakage of air or oil from the high-pressure oil chamber 44.
While the chain 43 is running, the plunger 43 of the foregoing conventional hydraulic tensioner 41 may be subjected to a force or load applied from the chain 62 in a direction to move the plunger 43 backward against the force of the spring 45 whereupon the hydraulic pressure in the high-pressure oil chamber 47 increases. In this instance, air or oil leaks from the orifice 56A of the orifice member 56 to thereby absorb an impact force applied to the plunger 43. At the same time, a pressure rise is developed in the pressure in the secondary oil chamber 50, which forces the sleeve 51 of the relief valve 54 to gradually move backward away from the secondary oil chamber 50 against the force of the spring 52. As the pressure in the secondary oil chamber 50 further increases, the sleeve 51 is caused to move backward past the discharge port 53 whereupon the discharge port 53 is opened, allowing the oil to flow out from the secondary oil chamber 50 to thereby relieve the pressure in the high-pressure oil chamber 44.
FIG. 6 shows in cross section another example of the conventional hydraulic tensioner which includes a relief valve and an orifice member provided separately.
The hydraulic tensioner 41xe2x80x2 is equipped with an oil supply passage 48 and a check valve mechanism 47, a plunger 43xe2x80x2, a high-pressure oil chamber 44xe2x80x2 and an orifice member 56 that are similar to those of the tensioner 41 shown in FIG. 5. The conventional tensioner 41xe2x80x2 is substantially the same in function and operation as the conventional tensioner shown in FIG. 5 but differs from the latter in the respective positions of the relief valve 54 and orifice member 56.
More specifically, the relief valve 54 is disposed inside the housing 42 and located above the high-pressure oil chamber 44. The orifice member 55 is provided at a tip end of the plunger 42.
The operation and function of the hydraulic tensioner 41xe2x80x2 are the same as those of the hydraulic tensioner 41 of FIG. 5 and a further description thereof can, therefore, be omitted.
As discussed above, in the foregoing examples of the conventional hydraulic tensioners, the relief valve 54 which is adapted to be opened to release the pressure of the high-pressure oil chamber beyond a predetermined limit, and the orifice member 56 which is designed to vent air or leak the oil from the high-pressure oil chamber to thereby absorb an impact force applied to the plunger are provided separately. Thus, it has been conventionally impossible to integrate these two functionally different components into a single unit. Since the relief valve and the orifice member are arranged separately, the conventional hydraulic tensioners are relatively large in size and hence require a large space for installation, and are expensive to manufacture.
It is accordingly a general object of the present invention to solve the foregoing problems associated with the prior art.
A more specific object of the present invention is to provide a hydraulic tensioner including a relief valve and an orifice member integrated into a single operating unit which is capable of achieving both a pressure-releasing function and a fluid-leaking function with respect to a high-pressure oil chamber, occupies only a small space in the housing and hence is contributive to the downsizing of the tensioner, and adds to reduction of the manufacturing cost of the tensioner.
To achieve the foregoing objects, there is provided, according to the present invention, a hydraulic tensioner comprising: a tensioner housing having a plunger accommodation bore formed therein, a plunger slidably fitted into the plunger accommodation bore of the tensioner housing and urged by a spring such that a tip portion of the plunger is projected to the exterior of the plunger accommodation bore, the plunger having a hollow portion opening to an end face of the plunger which faces a bottom wall of the plunger accommodation bore, there being a high-pressure oil chamber defined between the housing and the hollow portion of the plunger and filled with oil; a check valve mechanism equipped with a check ball for permitting the passage therethrough of the oil in only one direction from the exterior of the housing into the high-pressure oil chamber; and a relief valve mechanism arranged in fluid communication with the high-pressure oil chamber. The relief valve mechanism includes a generally cup-shaped orifice member having a peripheral orifice formed in an outer circumferential surface of the orifice member for allowing the oil to leak out from the high-pressure oil chamber, and a relief valve integrally assembled with the orifice member and solely operable to release the oil from the high-pressure oil chamber when the pressure in the high-pressure oil chamber exceeds a predetermined limit. The relief valve includes a cup-shaped sleeve slidably mounted in the hollow cylindrical orifice member, a sleeve spring urging the sleeve in a direction toward the high-pressure oil chamber, and a discharge port formed in a sidewall of the orifice member. The discharge port is normally closed by the sleeve and adapted to be opened when the sleeve is displaced in a direction away from the high-pressure oil chamber against the force of the sleeve spring.
With this arrangement, when the plunger is subjected to a force or load applied from a running chain in a direction to force the plunger backward against the force of the spring, the pressure in the high-pressure oil chamber increases as the check valve mechanism blocks the oil from flowing out from the high-pressure oil chamber. With this pressure rise developed in the high-pressure chamber, air or oil in the high-pressure oil chamber leaks from the peripheral orifice, to thereby absorb a shock force applied to the plunger. The pressure rise in the high-pressure oil chamber is transmitted to secondary oil chamber in which the sleeve is slidably received. When the hydraulic pressure applied to the sleeve exceeds the force of the sleeve spring, the sleeve starts moving backward against the force of the sleeve spring. A further increase in the pressure in the secondary oil pressure causes the sleeve to move backward past the discharge port whereupon the discharge port is opened, allowing the oil to flow out from the secondary oil chamber to thereby release the pressure of the high-pressure oil chamber. The oil leaked from the peripheral orifice and the oil flow out from the discharge port are discharged from the housing through a discharge hole.
In one preferred form of the present invention, the relief valve mechanism is assembled in a second hollow portion of the plunger formed contiguously with the hollow portion and having one end opening to a tip end of the plunger. The plunger includes an end plug fitted in the second hollow portion to close the one end thereof, and the end plug has a discharge hole for discharging the oil therefrom to the exterior of the tensioner after the oil has leaked from the orifice or has been released from the discharge port.
In another preferred form of the present invention, the relief valve mechanism is assembled in a mounting hole formed in the housing and having one end connected to the high-pressure oil chamber. The housing includes an end plug fitted in the mounting hole to close the other end thereof, and the end plug has a discharge hole for discharging the oil therefrom to the exterior of the tensioner after the oil has leaked from the orifice or has been released from the discharge port. For smooth and reliable venting of air from the high-pressure oil chamber, it is preferable that the relief valve mechanism is disposed above the high-pressure oil chamber.
Preferably, the cup-shaped orifice member has a bottom wall at an end thereof facing the high-pressure oil chamber, an oil passage formed in the bottom wall, and a secondary oil chamber formed interiorly of the cut-shaped orifice member and communicating with the high-pressure oil chamber through the oil passage. The cup-shaped sleeve is slidably received in the secondary oil chamber.